1. Field of the Invention
The present invention relates to an optical pickup apparatus that irradiates laser beam to a multi-layer optical disc having plural recording layers, and receives reflection light from the multi-layer optical disc, in order to reproduce a signal recorded on the multi-layer optical disc.
2. Description of the Related Art
In general, an optical pickup apparatus is used in an optical disc apparatus that records a signal on an optical disc, such as CD, DVD, BD (Blu-ray Disc), HD DVD, or the like, or reproduces a signal recorded on an optical disc. The optical pickup apparatus converges laser beam emitted from a laser source onto an optical disc by an objective lens, and receives the reflection light from the optical disc by a photodetector, so as to output an electrical signal according to the amount of received light.
In such an optical pickup apparatus, it has conventionally been demanded to increase a storage capacity. As a method for increasing the storage capacity of the optical disc, there are mainly two schemes, i.e., the enhancement of recording density and increase in the number of the recording layer. Of these schemes, the enhancement of recording density can be realized by increasing numerical aperture of the objective lens that converges laser beam onto the optical disc and shortening a wavelength of laser beam. However, increasing the numerical aperture of the objective lens and shortening the wavelength of laser beam are reaching their limit, and hence, it is extremely difficult to further increase the numerical aperture of the objective lens and shorten the wavelength of laser beam.
On the other hand, as for increasing the number of recording layer of the optical disc, a “two-layer disc” having two recording layers laminated on one substrate is standardized in, for example, a BD (Blu-ray Disc). The development of a multi-layer optical disc having more plural recording layers laminated has been expected. For example, Japanese Unexamined Patent Application No. 2004-355791 discloses an optical disc apparatus wherein various parameters, such as numerical aperture and tilt angle of optical axis of laser beam, are appropriately set corresponding to a multi-layer optical disc having five or more recording layers.
However, in the multi-layer optical disc, since a part of the laser beam irradiated to the recording layer, which is a subject of signal reproduction, is irradiated to the recording layers other than the aforesaid recording layer, an interlayer crosstalk occurs in which the reflection lights from the other recording layers are overlapped with the reflection light from the recording layer that is the subject of signal reproduction. In this case, the electrical signal outputted from the photodetector includes the signal recorded on the other recording layers due to the interlayer crosstalk, resulting in entailing a problem of deteriorating reproduction precision of the signal recorded on the recording layer that is the subject of signal reproduction. Since the interlayer crosstalk increase with the increase of the number of the recording layer, the interlayer crosstalk becomes a significant problem in a multi-layer optical disc that has been further developed and has more recording layers laminated.